Preheat the oven to 425°F (220°C).
In a large mixing bowl, combine the diced sweet potatoes, olive oil, chili powder, cumin, and salt. Toss to coat the sweet potatoes evenly.
Spread the sweet potatoes in a single layer on a baking sheet. Roast in the preheated oven for 25-30 minutes, or until tender and lightly browned, stirring halfway through cooking.
While the sweet potatoes are roasting, prepare the other ingredients. In a separate bowl, combine the black beans and corn. Squeeze half of the lime juice over the mixture and toss to combine.
Once the sweet potatoes are cooked, remove them from the oven and let them cool slightly.
To assemble the taco bowls, start with a base of shredded lettuce. Top with roasted sweet potatoes, black bean-corn mix, diced avocado, and cherry tomatoes.
Drizzle with the remaining lime juice, and sprinkle with fresh cilantro if desired. Serve with Greek yogurt or sour cream on the side.